The last half-a-century has seen great developments in olefin polymerization catalysis and particularly in the ability to modify polymer architecture and physical properties by controlling the structure of the catalyst. Design of well-defined, single-site catalysts has emerged as a powerful method to control polymer features such as tacticity, molecular weight, molecular weight distribution, and amount of co-monomer incorporation.
Early transition metal metallocene complexes have provided the most important and well-studied framework for single-site catalysts for olefin polymerization. (Coates, G. W. Chem. Rev. 2000, 100, 1223-1252. Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253-1345.) Recently, non-metallocene frameworks have emerged as versatile alternatives. (Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283-315. Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. Engl. 1999, 38, 428-447. Coates, G. W.; Hustad, P. D.; Reinartz, S. Angew. Chem., Int. Ed. Engl. 2002, 41, 2236-2257.) Complexes based on iron, cobalt, nickel and palladium have been shown to polymerize and oligomerize olefins with good activities and sometimes in a living fashion. (Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169-1203.)
In the realm of early transition metal polymerization catalysis, frameworks displaying an extensive range of multidentate ligands have been utilized. In this context, a broad interest has been shown in generating polymers with controlled microstructure through the use of non-metallocene catalysts. Promising advances have been made in both the development of single-site living polymerization catalysts and the design of ancillary ligands that have the appropriate symmetry for polymer tacticity control. The fundamental polymerization behaviors observed for individual systems are not yet well understood. Thus further exploration into the field of non-metallocene olefin polymerization catalysis is required. These frameworks also present the advantage of being relatively inexpensive and easy to both prepare and modify.
Anilides and phenolates are common anionic donors found in multidentate ligands for polymerization catalysis. Some of the most successful non-metallocene polymerization catalysts include bi-, tri-, and tetradentate anilide and phenolate ligands. Tridentate bisanilide ligands have been reported to support ethylene and α-olefin polymerization; in some cases living polymerization of 1-hexene was possible. (Mehrkhodavandi, P.; Schrock, R. R.; Pryor, L. L. Organometallics 2003, 22, 4569-4583.) Bidentate imino-phenolate ligands have been shown to support C2-symmetric architectures; these catalysts are able to generate syndiotactic or isotactic polypropylene depending on the nature of the substituents on the phenolate rings. (Mason, A. F.; Coates, G. W. J. Am. Chem. Soc. 2004, 126, 16326-16327. Mitani, M.; Furuyama, R.; Mohri, J.; Saito, J.; Ishii, S.; Terao, H.; Nakano, T.; Tanaka, H.; Fujita, T. J. Am. Chem. Soc. 2003, 125, 4293-4305.)
Tetradentate bisphenolate frameworks have been reported to give very active catalysts for the polymerization of 1-hexene; again, tacticity control was possible by use of C2-symmetric architectures. (Segal, S.; Goldberg, I.; Kol, M. Organometallics 2005, 24, 200-202.)
Tridentate bisphenolate frameworks have been successful as well in supporting olefin polymerization. (Takaoki, K.; Miyatake, T. Macromol. Symp. 2000, 157, 251-257. Nakayama, Y; Watanabe, K.; Ueyama, N.; Nakamura, A.; Harada, A.; Okuda, J. Organometallics 2000, 19, 2498-2503.)
With respect to olefin polymerization activity, a number of related systems have been investigated, based on the pyridine linker with phenoxides, alkoxides, or anilides as the anionic donors. (Chan, M. C. W.; Tam, K. H.; Pui, Y. L.; Zhu, N. Y. J. Chem. Soc., Dalton Trans. 2002, 3085-3087. Chan, M. C. W.; Tam, K. H.; Zhu, N. Y.; Chiu, P.; Matsui, S. Organometallics 2006, 25, 785-792. Mack, H.; Eisen, M. S. J. Chem. Soc., Dalton Trans. 1998, 917-921. Guerin, F.; McConville, D. H.; Vittal, J. J. Organometallics 1996, 15, 5586-5590. Gauvin, R. M.; Osborn, J. A.; Kress, J. Organometallics 2000, 19, 2944-2946.) It was found that a zirconium pyridine bisphenoxide system can polymerize ethylene with high activities and also incorporate propylene.
In this connection, a ligand involving a pyridine linker and two phenoxides was reported by to bind to iron(III), copper(II), and aluminum(III) in a C2 fashion. (Steinhauser, S.; Heinz, U.; Sander, J.; Hegetschweiler, K. Z. Anorg. Allg. Chem. 2004, 630, 1829-1838.) When bound to boron (Li, Y. Q.; Liu, Y; Bu, W. M.; Guo, J. H.; Wang, Y. Chem. Commun. 2000, 1551-1552.) or zirconium(IV) this ligand binds in a Cs fashion.
A chiral cationic zirconium pyridine bisalkoxide was found to insert only one ethylene molecule, (Gauvin, R. M.; Osborn, J. A.; Kress, J. Organometallics 2000, 19, 2944-2946.) while a related titanium pyridine bisalkoxide was reported to polymerize ethylene with good activity. (Mack, H.; Eisen, M. S. J. Chem. Soc., Dalton Trans. 1998, 917-921.) A zirconium pyridine bisanilide system was shown to polymerize ethylene upon activation with MAO. (Guerin, F.; McConville, D. H.; Vittal, J. J. Organometallics 1996, 15, 5586-5590.) Notably, computational studies on bisphenoxide-donor systems indicated that a strong interaction with the additional donor lowers the transition state for olefin insertion. (Froese, R. D. J.; Musaev, D. G.; Morokuma, K. Organometallics 1999, 18, 373-379.)